The invention is related to a deflection arrangement for separating ewe particle beams and a particle beam system with such a deflection arrangement.
In particle beam systems, especially in high-resolution B-beam systems, there is the task of separating illuminating particles (primary particle beam and signal particles (secondary particle beam) generated by the illuminating particle. This general task of separating two beans has to be performed both in scanning systems (like SEM""s) and in parallel imaging systems (like LEEM""s).
The following components are frequently used as beam separators in known arrangements:
magnetic sector fields,
electrostatic and magnetic deflectors in one, two or three stages and
Wien filter.
In surface imaging tools having a wider beam, complete image fields are illuminated with an illumination particle beam, and signal particles, e.g. secondary particles, back-scattered particles, reflected particles or other particles released at the specimen are imaged by a detector on which a complete image of the surface area is generated. N. Telieps, xe2x80x9cSurface Imaging with LEEMxe2x80x9d, Appl. Phys. A 44, 55-61, 1987, discloses such a surface imaging tool. The primary particle beam and the secondary particle beam are deflected by a magnetic sector field. The angle of incidence of the primary particle beam is a with respect to the optical axis and the beam will be deflected by the magnetic sector field onto the optical axis while the secondary particle beam reflected from the specimen rill be deflected to the ocher side of the optical axis to separate the secondary particle beam from the primary particle team.
Magnetic sector fields, however, introduce chromatic aberration, which limits the resolution. In order to keep the resolution reasonable, only small separation angles can be achieved.
EP-A-0 817 235 relates to a scanning electron microscope that obtains a scan image on the basis of secondary electrons emitted by irradiating an electron beam to a specimen. This microscope uses a Wien filter to separate the primary electron beam fray she secondary electrons. A Wien filter generates a crossed electro-static and magnetic deflection field, which, however, also introduces chromatic aberration. Only in the case that a crossover or an image of the surface is situated in the center of the Wien filter, chromatic aberration can be cancelled out. This however, restricts the optical design.
Electrostatic and magnetic deflectors can be arranged to an achromatic arrangement. This, however, requires at lease a two stage or three stage arrangement as disclosed in SP-A-0 937 177. Optical systems with many deflection system, however, extent the length of the optical system which has a negative influence on the particle bear interaction and consequently on the image resolution.
It is therefore an object of the invention to provide a deflection arrangement for separating two particle beams according to the preamble to claim 1 or a particle beam system with such a deflection arrangement according to the preamble to claim 8 which provides an achromatic operation and a good image resolution.
The object is solved by the features of claims 1 and 8.
Further embodiments of the invention are the subject matter of the subordinate claims.
According to the invention, the deflection arrangement for separating two particle beams has an electrostatic deflector and a magnetic deflector having a common optical axis and generating crossed electrostatic and magnetic deflection fields, wherein the two particle beams pass the deflection arrangement from opposite sides. The two deflectors are adapted to deflect one of the two beams achromatically by an angle xcex1 and the ocher beam by an angle xcex2xe2x89xa73xcex1 with respect to the angle of incidence, respectively.
In one embodiment of she invention, the deflection arrangement is positioned in a crossover of one of the two beams, preferably in a crossover of the beam which is not deflected achromatically. With such an arrangement, the chromatic aberration can be cancelled out for both beams.
In a preferred embodiment, the deflection fields of the electrostatic deflector and the magnetic deflector are superimposed, similar to an arrangement in a Wien filer. The major difference to a common Wien filter, however, is the excitation of the electrostatic and magnetic deflector:
1. The magnetic deflection is defined by:
xcex1m=CmHU0xe2x88x920.5xe2x80x83xe2x80x83(1)
dxcex1m=0.5*CmHU0xe2x88x921.5*dU0=xe2x88x920.5*xcex1mdU0xe2x80x83xe2x80x83(2)
dxcex1m/xcex1m=0.5 dU0/U0xe2x80x83xe2x80x83(3)
xcex1m=magnetic deflection angle
Cm=geometric data of the magnetic deflector
H=magnetic field strength
U0=acceleration voltage of the primary particle
dxcex1m=variation of the deflection angle by varying the primary energy by dU0 
2. The electrostatic deflection is defined by:
xcex1e=Ce*UD*U0xe2x88x921xe2x80x83xe2x80x83(4)
dxcex1e=xe2x88x92Ce*UD*U0xe2x88x921*dU0=xe2x88x92xcex1e*dU0xe2x80x83xe2x80x83(5)
dxcex1e/xcex1e=xe2x88x92dU0/U0xe2x80x83xe2x80x83(6)
xcex1e=electrostatic deflection angle
Ce=geometric data of the electrostatic deflector
H=magnetic field strength
U0=acceleration voltage of the primary particle
dxcex1m=variation of the deflection angle by varying the primary energy by dU0 
3. Superposition of electrostatic and magnetic deflector:
dxcex1=(xe2x88x92xcex1exe2x88x920.5xcex1m)dU0/U0xe2x80x83xe2x80x83(7)
dxcex1=variation of the total deflection angle of the superimposed electrostatic end magnetic deflects by varying the primary energy by dU0 
An achromatic deflector can be realized if dxcex1=0. This is true, if the excitation of the magnetic deflector is double as high in amplitude as that one of the electrostatic deflector. The deflection direction of both systems is opposite. The effect of such an arrangement is that the magnetic deflector tries to the deflect the beam by an angle of 2 xcex1, while the electrostatic deflector beds the beam back by xcex1. The resulting deflection power of this arrangement is xcex1. Since the chromatic aberration of the electrostatic deflector is twice as high as tat one of the magnetic deflector, the resulting deflection is achromatic, independent of the beam trace.
For the beam coming from the opposite direction, electrostatic and magnetic deflection forces are working in the same direction, which means that this beam is deflected by at least 3 xcex1. The total separation angle of the two beams is consequently at least 4 xcex1.
The achromatic deflection is only the for one beam. For the other beam, the energy dispersion of the two superimposed deflectors will even add. This, however, is in most cases no drawback, because resolution requirements for one of the two beams are of less importance.
In case that chromatic compensation is required for both beams, the arrangement and the corresponding excitation of the two deflection offers also a solution. In this case, the beam which is not achromatically deflected has to provide a crossover in the center of the deflection arrangement. In this case, the chromatic aberration of the second beam is also compensated.